Many experts believe that the placental barrier protects the fetus from harm.

But in 1960, many experts believed that the placental barrier shielded a fetus from harm. Kelsey's earlier animal-based research demonstrated the opposite: drugs could pass from mother to fetus through the placenta. Like other drug companies at this time, Merrell had not tested its drug on pregnant animals. Kelsey later said Merrell’s evidence for thalidomide’s safety seemed “more like testimonials than the results of well-designed studies.” Kelsey rejected Merrell’s application and asked them to submit a second backed by better evidence.